A sealing mechanism for a reactor vessel (RV) cable penetration tube improves the functional and structural integrity of a cable inserted in an RV through a penetration tube due to use of a precise thimble. The sealing mechanism includes a penetration tube configured to penetrate an RV from an outside to an inside thereof and having a penetration hole for communication with the inside of the RV, a cable configured to be inserted in the RV through the penetration hole of the penetration tube, and a thimble placed between the cable and the penetration tube, wherein a dimple groove portion is provided on the thimble in a direction from an outer surface of the penetration hole toward the cable.